(1) Field of the Invention
This invention relates to reducing the color of lignin. More particularly, this invention relates to a two-step process for reducing the color of lignin by first blocking the phenolic hydroxyl and then oxidizing the blocked lignin.
Lignin in wood is almost colorless. Lignin isolation procedures and pulping processes invariably introduce a variety of chromophores into the structure and render the isolated lignins strongly colored. Industrial lignins (such as sulfonated alkali lignins from the kraft and soda pulping processes and lignosulfonates from the sulfite pulping process) are colored to different degrees depending on the pulping conditions and the type of pulping process employed for delignification. The drastic conditions of a pulping process, e.g., high temperature and strong caustic in combination with atmospheric oxygen, produce lignin so intensively darkened that their uses in many application areas become objectionable because of the color.
The problem of lignin color is quite predominate in application areas where lignin is absorbed onto a substrate. Thus, for example, if lignin is used as a dispersant for dyestuff, it is usually ball-milled with a dye cake and the mixture then used for dyeing natural or synthetic fibers. One of the major disadvantages and undesirable effects of lignosulfonates and sulfonated alkali lignins as dyestuff dispersants is the staining of fabric fibers of which the degree is greatly dependent on the color of lignin. This so-called fiber staining caused by lignin tends to distort the authentic color of dye on the fabric.
The causes for the dark brown color of industrial lignins and the mechanisms for the formation of chromophores during the pulping processes are not completely known, although numerous suggestions have been made previously. For example, Gierer in Sv. Papperstidn 73 (1970) p. 561 proposed o,p'-stilbenequinone, p,p'-stilbenequinone, 1,4-bis-(p-hydroxyphenyl)-buta-1,3-diene, o-benzoquinone and methylene quinone as possible chromophoric structures. Kringstad et al. in TAPPI 54 (1971) p. 1680 estimated the amount of o-quinonoid structures in spruce milled-wood lignin to be about 0.7% and stated that the amount of quinonoid structures can account for as much as 35-60% of the light absorption of the lignin at 457 nm. The majority of chromophoric structures in alkali lignins and lignosulfonates appear to be some sort of conjugated systems involving quinonoid and side-chain double bonds.
(2) The Prior Art
The aforementioned conjugated systems may be cleaved by some oxidative processes or saturated by reductive processes to achieve some reduction of lignin color. Reductive processes change quinones to colorless catecholic structures which, however, are not stable under the influence of oxygen (air) and sunlight.
On the other hand, oxidative processes convert quinonoid structures to colorless aliphatic acids. The oxidative process also causes cleavage of unsaturated carbon-carbon bonds in the propanoid side chains of lignin molecules. By doing so, some extensively conjugated systems (chromophores) are destroyed, resulting in some reduction of lignin color. An advantage of the oxidative process is the fact that colorless end-products in oxidation reactions are stable and chromophores are not reformed thereof. However, uncontrolled oxidative conditions invite random destruction of lignin aromaticity and concurrently give rise to the formation of color bodies. For example, colored quinonoid moieties are produced in lignin by the following oxidative demethylation pathway: ##STR1##
It is also known that the color of lignin may be lightened to some degree by blocking the free-phenolic of hydroxyl in lignin. Several blocking methods have been set forth, such as in U.S. Pat. No. 3,672,817 wherein the lignin color was reduced as much as 44% by blocking the phenolic hydroxyl with an alkylene oxide or a halogen-containing alkyl alcohol. In U.S. Pat. No. 3,763,139, lignin color was reduced by blocking the phenolic hydroxyl with reagents, such as chloromethesulfonate, chloromethane phosphonate, 2-chloroethanol and the like. In U.S. Pat. No. 3,769,272, lignin color was reduced by blocking with 3-chloro-2-hydroxypropane-1-sulfonate. And in U.S. Pat. No. 3,865,803, the phenolic hydroxyl was blocked with an agent of the type X(CH.sub.2).sub.n Y, wherein X is a halogen, an activated double bond, an epoxide ring or a halohydrin, Y is a sulfonate, phosphonate, hydroxyl, sulfide or a secondary or tertiary amine and n is an integer from 1 to 5.
Although each of the described methods for some reduction of the color of an alkali lignin or lignosulfonate, none have reduced the color to a sufficiently desirable extent or to the extent of the process of this invention.
It is thus the general object of this invention to provide a two-step process whereby the color of sulfonated alkali lignins and lignosulfonates are drastically reduced.
Another object of this invention is to provide light colored lignin dyestuff dispersants which are practically non-staining on fabric.
Other objects, features and advantages of this invention will be seen in the following detailed description of the invention.